1. Field of the Invention
The superconducting thin film system of the present invention relates to superconducting structures in combination with fabrication of such structures for general but not exclusive use in the high end micro-wave spectrum for particular application to satellite communications, cellular communications, radar and missile seeking devices which require higher frequencies in the millimeter wavelength region. Such structures are particularly useful for incorporation of passive microwave components incorporated within a high temperature superconducting layer (HTSC) with such components including resistors, capacitors, delay lines, filters, as well as a number of other electronic components. In particular, this invention pertains to superconducting thin film systems which provide for low microwave loss superconducting films integrated with garnet substrates. Still further, this invention relates to a superconducting thin film system where a single crystal high temperature superconducting layer (HTSC) is integrated with a garnet substrate through a plurality of epitaxially grown and contiguously interfacing transitional buffer layers where the buffer layers provide for a lattice constant matching criteria between a lattice constant of the garnet substrate and the lattice constant of the high temperature superconducting layer. Further, this invention directs itself to a superconducting thin film System which provides for a garnet substrate having a lattice constant with a first epitaxially grown and contiguously interfacing perovskite compound buffer layer being deposited on an upper surface of the garnet substrate with the perovskite compound buffer layer having a lattice constant less than the lattice constant of the garnet substrate. Additionally, the superconducting thin film system includes a second perovskite compound buffer layer epitaxially grown and contiguously interfacing with an upper surface of the first perovskite buffer layer with the second perovskite compound buffer layer having a lattice constant less than the lattice constant of the first perovskite buffer layer and matched to or approximating the lattice constant of a single crystal high temperature superconducting layer deposited on an upper surface of the second perovskite buffer layer. In order for superconducting films to be useful at higher frequencies, substrate dielectric losses must be reduced which requires a substrate such as garnet having a low dielectric constant, and low loss tangent. However, garnet substrates have a relatively large lattice constant and have been found to be generally incompatible for integration with high temperature superconducting layers. The subject superconducting thin film system provides a structure and method of forming the same which integrates the garnet substrate layer with the high temperature superconducting film through a series of perovskite compound buffer layers which have monatonically decreasing lattice constant and are epitaxially grown on the garnet substrate and each to the other to provide an overall structure having a decreasing lattice constant throughout the layered system with a final perovskite layer having a lattice constant substantially matching the lattice constant of the high temperature superconducting film.
2. Prior Art
High temperature superconducting thin films used in microwave components must be deposited on a microwave compatible substrate having a low dielectric constant and a low loss tangent to avoid unacceptable power dissipation in the substrate. High temperature superconducting thin films have been deposited on a variety of substrates including alkaline earth fluoride substrates with success. However, garnet substrates are of great importance and have found many uses in microwave applications due to their optimum magnetic properties. However, attempts to grow high quality, high temperature superconducting thin films on garnet substrates have generally proved unsuccessful due to the fact that the films which were produced were found generally to be polycrystalline and had relatively low superconductive transition temperatures, and poor microwave properties.
The main considerations dictating against the integration of high temperature superconducting films with garnet substrates are generally directed to (1) interfacial reactions between the garnet substrates and the high temperature superconducting films which are brought to the fore by the extremely high processing temperatures required for the growth of certain high temperature superconducting films; (2) a second consideration dictating against the integration of the high temperature superconducting films with a garnet substrate is due to the fact that there is a lack of lattice matching between the high temperature superconducting films which generally have a lattice constant approximating 3.8 .ANG. to be grown on a garnet substrate having a lattice constant between 11.0-13.0 .ANG.. This lattice mismatch dictates against the growth of highly oriented films, since a plurality of orientations are favored during the growth process,
By use of buffer layers formed of perovskite compounds grown between the garnet substrate and the high temperature superconducting film, it has been found that a lattice matching technique and structure is formed which allows for highly oriented structures to be formed. By specifically growing perovskite compound layers having diminishing lattice constants between the garnet sublayer and the high temperature superconducting film, a highly oriented thin film superconducting layer is formed which is highly useful for microwave applications in the high frequency microwave spectrum range.